you could read Kernighan and Pike's "The Unix Programming Environment" and understand how to use and develop software for any of these platforms because underneath they were mostly the same code, and really did behave in mostly the same way. The UNIX kernel provided enough separation from the hardware to make it standard.
I went through lots of "The Unix Programming Environment" using DOS. People had ported/written unix tools for DOS that were similar enough to the Unix ones. I remember awk, sed, grep, lex, vi (elvis, stevie, calvin), emacs (freemacs, jove, miniemacs?), make, tar, gnuplot, shell. The GNU tools had the GNUish project, but didn't run on an 8088.
Microsoft was very focused on x86, and they even for a while owned Minix, a pretty good Unixlike for standard x86 PCs before they sold it to SCO.
Microsoft created Xenix and sold it to SCO. There was even an 8088 version.
Minix (https://en.wikipedia.org/wiki/Minix) was a very different beast created by Andrew Tanenbaum. It was a microkernel like Mach, created to teach systems programming to students. It cloned the Unix version 7 API and didn't need a Unix license (>$1000 at the time). Its unix tools were similar (or ported versions?) of the DOS tools I had used because their source was available and Minix only did the 64k I & D on 8088. Just like the DOS "small" model. The "proper" unix tools needed more RAM than that.
Anywho, TLDR Unix vendors didn't provide a common hardware platform, they provided a common software platform on which your code would compile and behave like you expected.
Yes. When I was a Unix sysadmin, we'd compile a whole suite of GNU and other tools for SunOS, Solaris, Irix, Ultrix, DEC OSF/1, HP-UX and AIX. There were lots of tiny API differences for each OS. Linux often just implemented more than one API to make porting easier. X11 was one example.
I don't miss chasing down dependencies back then. Linux really did packaging with Slakware, .deb and .rpm that the unixen didn't as much. Debian added dependency with apt and .rpm systems didn't really have it until yum came along.